Peer-Review Publications


A. V. Malyshev, D. A. Glazov, A. V. Volotka, I. I. Tupitsyn, V. M. Shabaev, G. Plunien, and T. Stöhlker
Ground-state ionization energies of boronlike ions
Phys. Rev. A, 96 :022512 (August 2017)
High-precision QED calculations of ground-state ionization energies are performed for all boronlike ions with nuclear charge numbers in the range 16≤Z≤96. Rigorous QED calculations are performed within the extended Furry picture and include all many-electron QED effects up to the second order of the perturbation theory. The contributions of third- and higher-order electron-correlation effects are accounted for within the Breit approximation. Nuclear recoil and nuclear polarization effects are taken into account as well. In comparison with previous evaluations of the ground-state ionization energies of boronlike ions the accuracy of the theoretical predictions is improved significantly.
D. Wu, and J. W. Wang
Magetostatic amplifier with tunable maximum by twisted-light plasma interactions
Plasma Phys. Contr. F., 59 :095010 (August 2017)
Laser beams with Laguerre–Gaussian (LG) mode carry orbital angular momentum (OAM); however, when interacting with plasmas, the net angular momentum acquired by plasmas is basically zero after interaction. Here, we find when there exists a small magetostatic seed along the laser propagation direction, the barrier would be broken, giving rise to dramatic angular momentum transfer from LG-lasers to plasmas. Hence, the net OAM remaining in the plasmas system would continuously enhance the magetostatic field, until the corresponding Larmor frequency of electrons is comparable to the laser frequency in vacuum. Three-dimensional particle-in-cell simulations are performed to confirm our theory, producing spatial-uniform, temporal-stable and extremely-intense magetostatic fields.
S. Fuchs, M. Wünsche, J. Nathanael, J. J. Abel, C. Rödel, J. Biedermann, J. Reinhard, U. Hübner, and G.G. Paulus
Optical coherence tomography with nanoscale axial resolution using a laser-driven high-harmonic source
Optica, 4 :903 (August 2017)
Extreme ultraviolet microscopy is technologically demanding and thus largely confined to synchrotron radiation facilities. However, specific benefits like high resolution and exceptional material contrast provide strong motivation for the development of table-top alternatives. We report on the first demonstration of coherence tomography, i.e., noninvasive cross-sectional imaging, with high harmonics. A depth resolution of 24 nm and very good material contrast are achieved. Excessively demanding optics for extreme ultraviolet radiation are avoided and artifacts due to the elementary geometry are suppressed with a novel three-step one-dimensional phase-retrieval algorithm. The images are recorded in reflection geometry, facilitating the analysis of, e.g., operating semiconductor samples.
A. A. Peshkov, D. Seipt, A. Surzhykov, and S. Fritzsche
Photoexcitation of atoms by Laguerre-Gaussian beams
Phys. Rev. A, 96 :023407 (August 2017)
In a recent experiment, Schmiegelow et al. [Nat. Commun. 7, 12998 (2016)] investigated the magnetic sublevel population of Ca^+ ions in a Laguerre-Gaussian light beam if the target atoms were just centered along the beam axis. They demonstrated in this experiment that the sublevel population of the excited atoms is uniquely defined by the projection of the orbital angular momentum of the incident light. However, little attention has been paid so far to the question of how the magnetic sublevels are populated when atoms are displaced from the beam axis by some impact parameter b. Here, we analyze this sublevel population for different atomic impact parameters in first-order perturbation theory and by making use of the density-matrix formalism. Detailed calculations are performed especially for the 4s ^2S_1/2 -> 3d ^2D_5/2 transition in Ca^+ ions and for the vector potential of a Laguerre-Gaussian beam in Coulomb gauge. It is shown that the magnetic sublevel population of the excited ^2D_5/2 level varies significantly with the impact parameter and is sensitive to the polarization, the radial index, as well as the orbital angular momentum of the incident light beam.
C. Scullion, D. Doria, L. Romagnani, A. Sgattoni, K. Naughton, D. R. Symes, P. McKenna, A. Macchi, M. Zepf, S. Kar, and M. Borghesi
Polarization Dependence of Bulk Ion Acceleration from Ultrathin Foils Irradiated by High-Intensity Ultrashort Laser Pulses
Phys. Rev. Lett., 119 :054801 (August 2017)
The acceleration of ions from ultrathin (10–100 nm) carbon foils has been investigated using intense (∼6×10²⁰ W cm⁻²) ultrashort (45 fs) laser pulses, highlighting a strong dependence of the ion beam parameters on the laser polarization, with circularly polarized (CP) pulses producing the highest energies for both protons and carbons (25−30  MeV/nucleon); in particular, carbon ion energies obtained employing CP pulses were significantly higher (∼2.5 times) than for irradiations employing linearly polarized pulses. Particle-in-cell simulations indicate that radiation pressure acceleration becomes the dominant mechanism for the thinnest targets and CP pulses.
Z. W. Wu, A. V. Volotka, A. Surzhykov, and S. Fritzsche
Angle-resolved x-ray spectroscopic scheme to determine overlapping hyperfine splittings in highly charged heliumlike ions
Phys. Rev. A, 96 :012503 (July 2017)
An angle-resolved x-ray spectroscopic scheme is presented for determining the hyperfine splitting of highly charged ions. For heliumlike ions, in particular, we propose to measure either the angular distribution or polarization of the 1s2p ^3P_1,F -> 1s^2 ^1S_0,F_f emission following the stimulated decay of the initial 1s 2s ^1S_0,F_i level. It is found that both the angular and polarization characteristics of the emitted x-ray photons strongly depend on the (relative) splitting of the partially overlapping hyperfine 1s 2p ^3P_1,F resonances and may thus help resolve their hyperfine structure. The proposed scheme is feasible with present-day photon detectors and allows a measurement of the hyperfine splitting of heliumlike ions with a relative accuracy of about 10^-4.
J. Andersson, R. Beerwerth, A. Roos, R. J. Squibb, R. Singh, S. Zagorodskikh, O. Talaee, D. Koulentianos, J. H. D. Eland, S. Fritzsche, and R. Feifel
Auger decay of 4d inner-shell holes in atomic Hg leading to triple ionization
Phys. Rev. A, 96 :012505 (July 2017)
Formation of triply ionized states upon the creation of 4d inner-shell holes in atomic Hg is investigated by using synchrotron radiation of 730 eV photon energy and a versatile multielectron coincidence detection technique in combination with multiconfiguration Dirac-Fock calculations. By carefully selecting Coster-Kronig electrons detected only in coincidence with a 4d photoelectron, the Coster-Kronig spectrum has been extracted and the corresponding branching ratios of the 4d hole have been determined. The results are found to differ from previously established experimental ratios based on electron impact ionization but to agree now better with theory. We also present an Auger cascade analysis of pathways leading to triply ionized states of atomic Hg upon removal of a 4d inner-shell electron.
J. Rothhardt, S. Hädrich, J. C. Delagnes, E. Cormier, and J. Limpert
High Average Power Near-Infrared Few-Cycle Lasers
Laser Photonics Rev., 11 :1700043 (July 2017)
Ultra-short laser pulses with only a few optical cycles duration have gained increasing importance during the recent decade and are currently employed in many laboratories worldwide. In addition, modern laser technology nowadays can provide few-cycle pulses at very high average power which advances established studies and opens exciting novel research opportunities. In this paper, the two complementary approaches for providing few-cycle pulses at high average power, namely optical parametric amplification and nonlinear pulse compression, are reviewed and compared. In addition, their limitations and future scaling potential are discussed. Furthermore, selected applications particularly taking advantage of the high average power and high repetition rate are presented.
M. Müller, A. Klenke, T. Gottschall, R. Klas, C. Rothhardt, S. Demmler, J. Rothhardt, J. Limpert, and A. Tünnermann
High-average-power femtosecond laser at 258 nm
Opt. Lett., 42 :2826 (July 2017)
We present an ultrafast fiber laser system delivering 4.6 W average power at 258 nm based on two-stage fourth-harmonic generation in beta barium borate (BBO). The beam quality is close to being diffraction limited with an M^2 value of 1.3×1.6. The pulse duration is 150 fs, which, potentially, is compressible down to 40 fs. A plain BBO and a sapphire-BBO compound are compared with respect to the achievable beam quality in the conversion process. This laser is applicable in scientific and industrial fields. Further scaling to higher average power is discussed.
H. Gies, and L. Zambelli
Non-Abelian Higgs models: Paving the way for asymptotic freedom
Phys. Rev. D, 96 :025003 (July 2017)
Asymptotically free renormalization group trajectories can be constructed in non-Abelian Higgs models with the aid of generalized boundary conditions imposed on the renormalized action. We detail this construction within the languages of simple low-order perturbation theory, effective field theory, as well as modern functional renormalization group equations. We construct a family of explicit scaling solutions using a controlled weak-coupling expansion in the ultraviolet, and obtain a standard Wilsonian renormalization group relevance classification of perturbations about scaling solutions. We obtain global information about the quasifixed function for the scalar potential by means of analytic asymptotic expansions and numerical shooting methods. Further analytical evidence for such asymptotically free theories is provided in the large-N limit. We estimate the long-range properties of these theories and identify initial/boundary conditions giving rise to a conventional Higgs phase.
J. Hofbrucker, A. V. Volotka, and S. Fritzsche
Photoelectron distribution of nonresonant two-photon ionization of neutral atoms
Phys. Rev. A, 96 :013409 (July 2017)
Photoelectron angular distributions following the nonresonant two-photon K-shell ionization of neutral atoms are studied theoretically. Using the independent particle approximation and relativistic second-order perturbation theory, the contributions of screening and relativistic effects to the photoelectron angular distribution are evaluated. A simple nonrelativistic expression is presented for the angle-differential cross section in dipole approximation for two-photon ionization by elliptically polarized photons, and its limitations are analyzed numerically. Moreover, we show that screening effects of the inactive electrons can significantly affect the photoelectron distributions and can also lead to a strong elliptical dichroism. Numerical results are presented for the case of two-photon K-shell ionization of neutral Ne, Ge, Xe, and U atoms.
M. Zürch, R. Jung, C. Späth, J. Tümmler, A. Guggenmos, D. Attwood, U. Kleineberg, H. Stiel, and C. Spielmann
Transverse Coherence Limited Coherent Diffraction Imaging using a Molybdenum Soft X-ray Laser Pumped at Moderate Pump Energies
Sci. Rep., 7 :5314 (July 2017)
Coherent diffraction imaging (CDI) in the extreme ultraviolet has become an important tool for nanoscale investigations. Laser-driven high harmonic generation (HHG) sources allow for lab scale applications such as cancer cell classification and phase-resolved surface studies. HHG sources exhibit excellent coherence but limited photon flux due poor conversion efficiency. In contrast, table-top soft X-ray lasers (SXRL) feature excellent temporal coherence and extraordinary high flux at limited transverse coherence. Here, the performance of a SXRL pumped at moderate pump energies is evaluated for CDI and compared to a HHG source. For CDI, a lower bound for the required mutual coherence factor of |μ12| ≥ 0.75 is found by comparing a reconstruction with fixed support to a conventional characterization using double slits. A comparison of the captured diffraction signals suggests that SXRLs have the potential for imaging micron scale objects with sub-20 nm resolution in orders of magnitude shorter integration time compared to a conventional HHG source. Here, the low transverse coherence diameter limits the resolution to approximately 180 nm. The extraordinary high photon flux per laser shot, scalability towards higher repetition rate and capability of seeding with a high harmonic source opens a route for higher performance nanoscale imaging systems based on SXRLs.
H. Stark, M. Müller, M. Kienel, A. Klenke, J. Limpert, and A. Tünnermann
Electro-optically controlled divided-pulse amplification
Opt. Express, 25 :13494 (June 2017)
A novel technique for divided-pulse amplification is presented in a proof-of-principle experiment. A pulse burst, cut out of the pulse train of a mode-locked oscillator, is amplified and temporally combined into a single pulse. High combination efficiency and excellent pulse contrast are demonstrated. The system is mostly fiber-coupled, enabling a high interferometric stability. This approach provides access to the amplitude and phase of the individual pulses in the burst to be amplified, potentially allowing the compensation of gain saturation and nonlinear phase mismatches within the burst. Therefore, this technique enables the scaling of the peak power and pulse energy of pulsed laser systems beyond currently prevailing limitations.
W. Cayzac, A. Frank, A. Ortner, V. Bagnoud, M. M. Basko, S. Bedacht, C. Bläser, A. Blazevic, S. Busold, O. Deppert, J. Ding, M. Ehret, P. Fiala, S. Frydrych, D. O. Gericke, L. Hallo, J. Helfrich, D. Jahn, E. Kjartansson, A. Knetsch, D. Kraus, G. Malka, N. W. Neumann, K. Pépitone, D. Pepler, S. Sander, G. Schaumann, T. Schlegel, N. Schroeter, D. Schumacher, and M. Seibert
Experimental discrimination of ion stopping models near the Bragg peak in highly ionized matter
Nat. Commun., 8 :15693 (June 2017)
The energy deposition of ions in dense plasmas is a key process in inertial confinement fusion that determines the α-particle heating expected to trigger a burn wave in the hydrogen pellet and resulting in high thermonuclear gain. However, measurements of ion stopping in plasmas are scarce and mostly restricted to high ion velocities where theory agrees with the data. Here, we report experimental data at low projectile velocities near the Bragg peak, where the stopping force reaches its maximum. This parameter range features the largest theoretical uncertainties and conclusive data are missing until today. The precision of our measurements, combined with a reliable knowledge of the plasma parameters, allows to disprove several standard models for the stopping power for beam velocities typically encountered in inertial fusion. On the other hand, our data support theories that include a detailed treatment of strong ion-electron collisions.
A. Alejo, A. G. Krygier, H. Ahmed, J. T. Morrison, R. J. Clarke, J. Fuchs, A. Green, J. S. Green, D. Jung, A. Kleinschmidt, Z. Najmudin, H. Nakamura, P. Norreys, M. Notley, M. Oliver, M. Roth, L. Vassura, M. Zepf, M. Borghesi, R. R. Freeman, and S. Kar
High flux, beamed neutron sources employing deuteron-rich ion beams from D2O-ice layered targets
Plasma Phys. Contr. F., 59 :064004 (June 2017)
A forwardly-peaked bright neutron source was produced using a laser-driven, deuteron-rich ion beam in a pitcher-catcher scenario. A proton-free ion source was produced via target normal sheath acceleration from Au foils having a thin layer of D₂O ice at the rear side, irradiated by sub-petawatt laser pulses (~200 J, ~750 fs) at peak intensity ∼2×10²⁰ W cm⁻². The neutrons were preferentially produced in a beam of ~70° FWHM cone along the ion beam forward direction, with maximum energy up to ~40 MeV and a peak flux along the axis ∼2×10⁹ n sr⁻¹ for neutron energy above 2.5 MeV. The experimental data is in good agreement with the simulations carried out for the d(d,n)³He reaction using the deuteron beam produced by the ice-layered target.
M. Chemnitz, M. Gebhardt, C. Gaida, F. Stutzki, J. Kobelke, J. Limpert, A. Tünnermann, and M. Schmidt
Hybrid soliton dynamics in liquid-core fibres
Nat. Commun., 8 :42 (June 2017)
The discovery of optical solitons being understood as temporally and spectrally stationary optical states has enabled numerous innovations among which, most notably, supercontinuum light sources have become widely used in both fundamental and applied sciences. Here, we report on experimental evidence for dynamics of hybrid solitons—a new type of solitary wave, which emerges as a result of a strong non-instantaneous nonlinear response in CS2-filled liquid-core optical fibres. Octave-spanning supercontinua in the mid-infrared region are observed when pumping the hybrid waveguide with a 460 fs laser (1.95 μm) in the anomalous dispersion regime at nanojoule-level pulse energies. A detailed numerical analysis well correlated with the experiment uncovers clear indicators of emerging hybrid solitons, revealing their impact on the bandwidth, onset energy and noise characteristics of the supercontinua. Our study highlights liquid-core fibres as a promising platform for fundamental optics and applications towards novel coherent and reconfigurable light sources.
S. H. Hendi, B. E. Panah, S. Panahiyan, and M. Momennia
Magnetic brane solutions in Gauss–Bonnet–Maxwell massive gravity
Phys. Lett. B, 772 :43 (June 2017)
Magnetic branes of Gauss–Bonnet–Maxwell theory in the context of massive gravity is studied in detail. Exact solutions are obtained and their interesting geometrical properties are investigated. It is argued that although these horizonless solutions are free of curvature singularity, they enjoy a cone-like geometry with a conic singularity. In order to investigate the effects of various parameters on the geometry of conic singularity, its corresponding deficit angle is studied. It will be shown that despite the effects of Gauss–Bonnet gravity on the solutions, deficit angle is free of Gauss–Bonnet parameter. On the other hand, the effects of massive gravity, cosmological constant and electrical charge on the deficit angle will be explored. Also, a brief discussion related to possible geometrical phase transition of these topological objects is given.
P. Finetti, H. Höppner, E. Allaria, C. Callegari, F. Capotondi, P. Cinquegrana, M. Coreno, R. Cucini, M. Danailov, A. Demidovich, G. De Ninno, M. Di Fraia, R. Feifel, E. Ferrari, L. Fröhlich, D. Gauthier, T. Golz, C. Grazioli, Y. Kai, G. Kurdi, N. Mahne, M. Manfredda, N. Medvedev, I. Nikolov, E. Pedersoli, G. Penco, O. Plekan, M. Prandolini, K. Prince, L. Raimondi, P. Rebernik, R. Riedel, E. Roussel, P. Sigalotti, R. Squibb, N. Stojanovic, S. Stranges, C. Svetina, T. Tanikawa, U. Teubner, V. Tkachenko, S. Toleikis, M. Zangrando, B. Ziaja, F. Tavella, and L. Giannessi
Pulse Duration of Seeded Free-Electron Lasers
Phys. Rev. X, 7 :021043 (June 2017)
The pulse duration, and, more generally, the temporal intensity profile of free-electron laser (FEL) pulses, is of utmost importance for exploring the new perspectives offered by FELs; it is a nontrivial experimental parameter that needs to be characterized. We measured the pulse shape of an extreme ultraviolet externally seeded FEL operating in high-gain harmonic generation mode. Two different methods based on the cross-correlation of the FEL pulses with an external optical laser were used. The two methods, one capable of single-shot performance, may both be implemented as online diagnostics in FEL facilities. The measurements were carried out at the seeded FEL facility FERMI. The FEL temporal pulse characteristics were measured and studied in a range of FEL wavelengths and machine settings, and they were compared to the predictions of a theoretical model. The measurements allowed a direct observation of the pulse lengthening and splitting at saturation, in agreement with the proposed theory.
F. Tavella, H. Höppner, V. Tkachenko, N. Medvedev, F. Capotondi, T. Golz, Y. Kai, M. Manfredda, E. Pedersoli, M. J. Prandolini, N. Stojanovic, T. Tanikawa, U. Teubner, S. Toleikis, and B. Ziaja
Soft x-ray induced femtosecond solid-to-solid phase transition
HEDP, 24 :22 (June 2017)
Abstract Soft x-rays were applied to induce graphitization of diamond through a non-thermal solid-to-solid phase transition. This process was observed within poly-crystalline diamond with a time-resolved experiment using ultrashort soft x-ray pulses of duration 52.5 fs and cross correlated by an optical pulse of duration 32.8 fs. This scheme enabled for the first time the measurement of a phase transition on a timescale of ∼150 fs. Excellent agreement between experiment and theoretical predictions was found, using a dedicated code that followed the non-equilibrium evolution of the irradiated diamond including all transient electronic and structural changes. These observations confirm that soft x-rays can induce a non-thermal ultrafast solid-to-solid phase transition on a hundred femtosecond timescale.
D. Zille, D. Seipt, M. Möller, S. Fritzsche, G.G. Paulus, and D. B. Milošević
Spin-dependent quantum theory of high-order above-threshold ionization
Phys. Rev. A, 95 :063408 (June 2017)
The strong-field-approximation theory of high-order above-threshold ionization of atoms is generalized to include the electron spin. The obtained rescattering amplitude consists of a direct and exchange part. On the examples of excited He atoms as well as Li^+ and Be^2+ ions, it is shown that the interference of these two amplitudes leads to an observable difference between the photoelectron momentum distributions corresponding to different initial spin states: Pronounced minima appear for singlet states, which are absent for triplet states.
V. Bagnoud, J. Hornung, T. Schlegel, B. Zielbauer, C. Brabetz, M. Roth, P. Hilz, M. Haug, J. Schreiber, and F. Wagner
Studying the Dynamics of Relativistic Laser-Plasma Interaction on Thin Foils by Means of Fourier-Transform Spectral Interferometry
Phys. Rev. Lett., 118 :255003 (June 2017)
We apply Fourier-transform spectral interferometry (FTSI) to study the interaction of intense laser pulses with ultrathin targets. Ultrathin submicrometer-thick solid CH targets were shot at the PHELIX laser facility with an intensity in the mid to upper 10^19  W/cm2 range using an innovative double-pulse structure. The transmitted pulse structure was analyzed by FTSI and shows a transition from a relativistic transparency-dominated regime for targets thinner than 500 nm to a hole-boring-dominated laser-plasma interaction for thicker targets. The results also confirm that the inevitable preplasma expansion happening during the rising slope of the pulse, a few picoseconds before the maximum of the pulse is reached, cannot be neglected and plays a dominant role in laser-plasma interaction with ultrathin solid targets.
M. Vockert, G. Weber, U. Spillmann, T. Krings, M. Herdrich, and T. Stöhlker
Commissioning of a Si(Li) Compton polarimeter with improved energy resolution
Nucl. Instr. Meth. Phys. Res. B, 408 :313 (May 2017)
Abstract On the basis of a double-side segmented Si(Li) crystal a new Compton polarimeter was developed within the SPARC collaboration. The new detector is equipped with a cryogenic first stage of the preamplifiers to improve the energy resolution compared to previous detectors with preamplifiers operating at room temperature. We present first results from a commissioning measurement of the new instrument at the ESR storage ring of GSI in Darmstadt, Germany and contrast it with the performance of an precursor polarimeter system.
B. Goswami, B. Antony, and S. Fritzsche
Electron impact scattering and calculated ionization cross sections for SFx (x=1–5) radicals
\u200eInt. J. Mass Spectrom., 417 :8 (May 2017)
Abstract The spherical complex optical potential (SCOP) formalism is employed to solve the e−-SFx (x=1–5) scattering system. In this article, the total cross sections by electron impact from 50 to 5000eV are calculated. The complex scattering potential ionization contribution (CSP-ic) method is used to compute the electron-induced total ionization cross sections from the inelastic cross section in the energy range from ionization threshold to 5000eV. For most of the reported radicals, the magnitude and shape of cross section compares well with previous measurements and calculations, wherever available. However, for many targets results are predicted for the first time in this work. From the electron-impact scattering cross sections for the SFx (x=1–5) radicals, we also estimate the gas-kinetic radius and the van der Waals coefficient.
M. Herdrich, G. Weber, A. Gumberidze, Z. Wu, and T. Stöhlker
Fast calculator for X-ray emission due to Radiative Recombination and Radiative Electron Capture in relativistic heavy-ion atom collisions
Nucl. Instr. Meth. Phys. Res. B, 408 :294 (May 2017)
Abstract In experiments with highly charged, fast heavy ions the Radiative Recombination (RR) and Radiative Electron Capture (REC) processes have significant cross sections in an energy range of up to a few GeV / u . They are some of the most important charge changing processes in collisions of heavy ions with atoms and electrons, leading to the emission of a photon along with the formation of the ground and excited atomic states. Hence, for the understanding and planning of experiments, in particular for X-ray spectroscopy studies, at accelerator ring facilities, such as FAIR, it is crucial to have a good knowledge of these cross sections and the associated radiation characteristics. In the frame of this work a fast calculator, named RECAL, for the RR and REC process is presented and its capabilities are demonstrated with the analysis of a recently conducted experiment at the Experimental Storage Ring (ESR) at the GSI Helmholtz Center for Heavy Ion Research in Darmstadt, Germany. A method is presented to determine unknown X-ray emission cross sections via normalization of the recorded spectra to REC cross sections calculated by RECAL.
J. Ullmann, Z. Andelkovic, C. Brandau, A. Dax, W. Geithner, C. Geppert, C. Gorges, M. Hammen, V. Hannen, S. Kaufmann, K. König, Y. Litvinov, M. Lochmann, B. Maass, J. Meisner, T. Murböck, R. Sanchez, M. Schmidt, S. Schmidt, M. Steck, T. Stöhlker, R. Thompson, C. Trageser, J. Vollbrecht, C. Weinheimer, and W. Nörtershäuser
High precision hyperfine measurements in Bismuth challenge bound-state strong-field QED
Nat. Commun., 8 :15484 (May 2017)
Electrons bound in highly charged heavy ions such as hydrogen-like bismuth 209^Bi^82+ experience electromagnetic fields that are a million times stronger than in light atoms. Measuring the wavelength of light emitted and absorbed by these ions is therefore a sensitive testing ground for quantum electrodynamical (QED) effects and especially the electron–nucleus interaction under such extreme conditions. However, insufficient knowledge of the nuclear structure has prevented a rigorous test of strong-field QED. Here we present a measurement of the so-called specific difference between the hyperfine splittings in hydrogen-like and lithium-like bismuth 209^Bi^82+,80+ with a precision that is improved by more than an order of magnitude. Even though this quantity is believed to be largely insensitive to nuclear structure and therefore the most decisive test of QED in the strong magnetic field regime, we find a 7-σ discrepancy compared with the theoretical prediction.